Many people check a sleep score on their smartwatch every morning. But how correct are these devices? In a 2021 US study, 34 young adults slept in a sleep lab with seven different trackers. The trackers were good at finding sleep.

But they often missed the time when people were awake and lying still. Some people worry too much about their scores and may then sleep worse. This is called orthosomnia.

Sleep trackers are popular, but their numbers are not always accurate. In a 2021 study, 34 healthy young adults slept in a laboratory for three nights while wearing or using seven different consumer devices. At the same time, their sleep was measured with polysomnography, a medical test that records brain waves.

All the devices correctly identified more than 93 percent of sleep, but they recognized only 18 to 54 percent of the time people were awake. This is because most trackers estimate sleep from body movement and heart rate, so lying still while awake looks like sleep. In 2017, doctors reported patients whose worries about tracker data seemed to make their insomnia worse. They called this problem orthosomnia.

Wearable sleep trackers promise insight into our nights, but their accuracy has clear limits, and paying too much attention to them can backfire. In a 2021 validation study, 34 healthy young adults spent three nights in a sleep laboratory using seven consumer devices, including wrist-worn and bedside models, while their sleep was simultaneously recorded by polysomnography. Every device showed high sensitivity, correctly classifying over 93 percent of sleep, but specificity, the ability to detect wakefulness, ranged from only 18 to 54 percent, and the classification of sleep stages varied widely between devices.

Performance also tended to worsen on nights when sleep was disrupted. Because most trackers infer sleep from movement and heart rate rather than brain activity, quiet wakefulness is easily mistaken for sleep. In 2017, clinicians described patients whose preoccupation with tracker data seemed to intensify their insomnia, for example by spending longer in bed to raise their tracked sleep time, and coined the term orthosomnia. It should be noted that the validation study involved only healthy young adults, and because manufacturers frequently update their algorithms, results for specific devices may quickly become outdated.